Water

Water is a clear liquid made of tiny particles called molecules. Each water molecule is built from three atoms: two hydrogen atoms and one oxygen atom. That is why scientists write water as H2O. Water covers about 71 percent of Earth's surface. It is found in oceans, lakes, rivers, clouds, ice, and inside every living thing.

Water is the only common substance on Earth that exists naturally in three different forms. As a liquid, it flows out of a faucet or falls as rain. As a solid, it freezes into ice or snow. As a gas, it floats invisibly in the air as water vapor. Water changes between these forms as it heats up or cools down. This is called changing state.

At sea level, water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit. Most substances get smaller and heavier when they freeze. Water does the opposite. Ice is less dense than liquid water, which is why ice cubes float instead of sinking. This tiny fact has huge effects. When a lake freezes, the ice floats on top and helps protect the fish below. If ice sank, many lakes would freeze solid in winter and most of the life in them would die.

Water is sometimes called the universal solvent. That means it can dissolve more kinds of substances than almost any other liquid. Salt, sugar, and many minerals break apart in water. Your blood is mostly water, and it carries dissolved food and oxygen to every cell in your body.

Living things cannot survive without water. A human body is about 60 percent water. A jellyfish is about 95 percent water. Even a dry-looking tree is nearly half water inside. You can live for weeks without food, but only about three days without water.

Earth has a lot of water, but not much of it is easy to drink. About 97 percent of Earth's water is salty ocean water. Only 3 percent is fresh water, and most of that is frozen in glaciers and ice caps. Less than 1 percent of all the water on Earth is liquid fresh water that people and animals can use. Clean drinking water is one of the most precious resources on the planet.